Method of searching code sequence in mobile communication system

ABSTRACT

A method for transmitting a signal for cell searching in a mobile communication system having a multi-cell environment includes transmitting the signal to one or more receiving party devices within a cell, wherein the signal is used for a synchronization of the one or more receiving party devices within the cell, the signal is defined by a combination of a first code sequence derived from a first index and a second code sequence derived from a second index, and an identity of the cell is used for defining the combination of the first code sequence and the second code sequence.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/915,340 filed on Jun. 11,2013, now U.S. Pat. No 9,161,295, witch is acontinuation of U.S. patent application Ser. No. 12/280,540, filed onJan. 6, 2009, now U.S. Pat. No. 8,483,036, witch is the National Stagefiling under 35 U.S.C. 371 of International Application No.PCT/KR2007/000973, filed on Feb. 26, 2007, which claims the benefit ofearlier filing date and right of priority to Korean Application Nos.10-2006-0031534, filed on Apr. 6, 2006, and 10-2006-0018103, filed onFeb. 24, 2006, the contents of which are all incorporated by referenceherein in their entirety.

TECHNICAL FIELD

The present invention relates to a mobile communication system, and moreparticularly, to a method of searching a code sequence in a cell basedmobile communication system, by which a receiving side is able toefficiently search code sequences.

BACKGROUND ART

Generally, in a mobile communication system, a mobile station uses apreamble which is broadcasted and used for searching for a cell to whichthe mobile station itself belongs. In order to effectively search cellsusing the preamble, various codes are used, which can be implemented byputting a limitation on a frequency or a spreading code, or the likethat is to be used.

In the above methods, a cell search performance depends on a sequenceitself. As an example of a sequence having a good autocorrelationcharacteristic and peak to average power ratio (hereinafter abbreviatedPAPR), there is CAZAC (constant modulus and zero autocorrelation)sequence. A mobile station is able to effectively search a cell usingthe CAZAC sequence. However, in an environment where other neighborcells are simultaneously detected like a cell boundary area, the mobilestation is unable to search a neighbor cell with simple algorithm buthas to perform a full search by calculating correlation for a sequencecorresponding to the entire cells.

FIG. 1 is a diagram of a cell search process according to a related art.Referring to FIG. 1, a cell transmits a preamble 11 to enable mobilestations to discriminate cells before transmitting data or controlinformation. In case that a mobile station attempts to communicate withthe cell, it receives the preamble 11 transmitted from the cell and thencarries out channel estimation using the received preamble 11.

In order to discriminate cells by receiving a preamble, the mobilestation is provided with a code set consisting of codes respectivelycorresponding to the cells and performs a cell search using signalstransferred by a correlator 13 and correlation values. In the above cellsearch method, cell search algorithm should be simple to effectivelyperform the cell search.

FIG. 2 is a structural diagram of a frame applied to a communicationsystem. First of all, assuming that a preamble code set transmittable bya cell is named C_(p) and that a preamble code assigned to a j^(th) cellis named C_(j), the cell transmits the preamble code before broadcastingcontrol information or data and then additionally transmits informationfor enabling precise channel estimation or synchronization.

A receiving side obtains signals of a received preamble part and thenchecks a correlation value with a preamble set provided to the receivingside. Equation 1 related to a correlation value checking method.

$\begin{matrix}{{J_{k}(\tau)} = {\sum\limits_{\tau = 0}^{N - 1}{c_{k}^{n^{*}}c_{j}^{n - \tau}}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, C_(k) indicates an arbitrary code existing in a code set,N indicates a code length, and τ indicates a delay component ofautocorrelation. In this case, a cost JM_(k)(τ) for C_(k) can beexpressed as Equation 2.

$\begin{matrix}{{JM}_{k} = {\max\limits_{{\tau = 0},\ldots\mspace{11mu},{N - 1}}{J_{k}(\tau)}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

If the cost is calculated for each code constructing the code set and ifa code having a maximum value among the calculated costs is selected, itis able to search a cell corresponding to the selected code. A cellsearch method using the cell cost is represented as Equation 3.

$\begin{matrix}{K^{*} = {\arg\;{\max\limits_{{k = 0},\ldots\mspace{11mu},{N - 1}}{JM}_{k}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In case that a code itself fails to have any configuration, all thecodes configuring the code set should be searched. Yet, in case that acode itself has a specific pattern, it is able to efficiently search acell using it. For instance, in case that CAZAC sequence is used as apreamble code, it is able to use a linear frequency increasecharacteristic of the CAZAC sequence. The frequency increasecharacteristic shows a single frequency component if a progression ofdifferences of a phase is calculated, if the calculated progression ofdifferences is converted to an exponential function value, and ifdiscrete Fourier transform (hereinafter abbreviated DFT) is performed onthe exponential function value. Yet, a result of the DFT in thecharacteristics of the CAZAC sequence is totally changed according tothe influence made by a different CAZAC sequence coming from anothercell in the vicinity of a cell boundary. Accordingly, in case of usingsuch a sequence as CAZAC, it is able to obtain a transmitted preamblecode through simple algorithm if a peripheral interference signal doesnot exist. If an interference signal however exists, the characteristicsof the CAZAC sequence are ruined. Accordingly, it is difficult to detectthe preamble through simple algorithm.

In particular, in case that a mobile station searches all possible codecombinations, it is unable to efficiently search a code sequence. Incase of such a sequence having a specific characteristic as a CAZACsequence, if interference signals considerably exist around a cellboundary, it is unable to search a code sequence using simple algorithm.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention is directed to a method of searchinga code sequence in mobile communication system that substantiallyobviates one or more of the problems due to limitations anddisadvantages of the related art.

An object of the present invention is to provide a method of enabling areceiving side to efficiently search a code sequence.

Another object of the present invention is to provide a method ofenabling fast initial acquisition by reducing complexity of a receivingside for a code sequence search and by reducing a cell search time.

Another object of the present invention is to provide a method ofreducing a cell search error.

Another object of the present invention is to provide a method ofexpanding code sequence types available for a mobile communicationsystem.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims thereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a method oftransmitting a code sequence for an efficient code sequence search in amobile communication system comprises boosting at least one tone for acode sequence, and transmitting the code sequence having the boosted atleast one tone to a receiving side.

Preferably, the at least one tone boosting step comprises increasing anamplitude of a code element corresponding to the at least one tone inthe code sequence, and modulating the code sequence through a pluralityof sub-carriers.

Preferably, the amplitude of the code element is increased in a mannerof boosting the at least one tone while an overall power allocated tothe code sequence is uniformly maintained.

Preferably, a specific one of the at least one tone establishes aprescribed relation with code identification information for identifyingthe code sequence in a code sequence set including the code sequence.

As another aspect of the present invention, a method of receiving a codesequence for an efficient code sequence search in a mobile communicationsystem comprises receiving signals for a code sequence having at leasttwo tones boosted by a transmitting side, and obtaining prescribedinformation from at least one of identification information for the codesequence and a combination of identification information for the boostedat least two tones of the code sequence.

As further another aspect of the present invention, a method ofreceiving a code sequence for an efficient code sequence search in amobile communication system comprises receiving signals for a codesequence having at least two tones boosted by a transmitting side andobtaining prescribed information previously agreed between thetransmitting side and a receiving side from at least one ofidentification information for the code sequence and a combination ofidentification information for the boosted at least two tones of thecode sequence.

As further another aspect of the present invention, a method of signalprocessing for an efficient code sequence search in a mobilecommunication system comprises increasing an amplitude of a code elementof a code sequence for cell search and modulating the code sequencethrough a plurality of sub-carriers. Preferably, amplitudes of othercode elements except the code element are adjusted to uniformly maintaina total power allocated to the code sequence.

As further another aspect of the present invention, a method oftransmitting a code sequence for cell search in a cell based mobilecommunication system comprises transmitting a first code sequence foridentifying a cell group including at least one cell to a receiving sideand transmitting a second code sequence for identifying a specific cellincluded in the cell group identified by the first code sequence to thereceiving side, wherein a specific tone of at least one of the first andsecond code sequences is boosted to be transmitted.

As further another aspect of the present invention, a method of cellsearch in a cell based mobile communication system comprises receivingsignals for a code sequence of which a tone is boosted, the signalstransmitted from a transmitting side, obtaining code identificationinformation for identifying the code sequence from elementidentification information for identifying a code element correspondingto the boosted tone, and identifying a cell using the codeidentification information.

As further aspect of the present invention, a method of cell search in acell based mobile communication system comprises receiving signals for acode sequence having a tone boosted by a transmitting side, obtaining acell group identifier for identifying a cell group including at leastone cell from element identification information for identifying a codeelement corresponding to the boosted tone, and identifying a cellincluded within the cell group using the code sequence.

As further aspect of the present invention, a method of cell search in acell based mobile communication system comprises obtaining a cell groupidentifier for identifying a cell group including at least one cellusing a first code sequence transmitted from a transmitting side andobtaining a cell identifier for identifying a cell included in the cellgroup identified by the cell group identifier using a second codesequence transmitted from the transmitting side, wherein thetransmitting side boosts a specific tone of at least one of the firstand second sequences.

As further aspect of the present invention, a transmitter for a mobilecommunication system comprises means for adjusting an amplitude of acode element for boosting a specific tone in a specific code sequenceand means for converting the code sequence into time domain signals.

As further aspect of the present invention, a method of transmittingsignals for cell search in a cell based mobile communication systemcomprises configuring a preamble by combining at least one code sequenceselected from a first code set and at least one code sequence selectedfrom a second code set together, the preamble transmitted ahead oftransmission of control signals and data signals and transmitting theconfigured preamble.

As further aspect of the present invention, a method of cell search in acell based mobile communication system comprises receiving a preambleincluding a combination of code sequences selected from at least twocode sets, respectively and performing cell search by usingcharacteristics of the code sequences configuring the combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a cell search process according to a related art.

FIG. 2 is a configurational diagram of a frame structure applied to acommunication system according to a related art.

FIG. 3 is a block diagram according to one preferred embodiment of thepresent invention.

FIG. 4 is an exemplary diagram of a case where tones of symbols (l=10,l=3, l=59) are boosted using a CAZAC sequence (M=1) for three symbolsaccording to one preferred embodiment of the present invention.

FIG. 5A and FIG. 5B are diagrams of amplitudes on frequency and timedomains for 520 sequence types generated in single-tone added constantamplitude zero auto correlation (SA-CAZAC) when Ng=521 and λ=3 accordingto the present invention.

FIG. 6A and FIG. 6B are diagrams of CDF of circular cross-correlationfor available 520 sequences according to a variation of λ for Ng=521 inSA-CAZAC according to one embodiment of the present invention.

FIG. 7A and FIG. 7B are diagrams of circular cross-correlation between asequence having 0^(th) tone boosted in a CAZAC sequence (M=10) and 520sequences having the rest of the boosted indexes.

FIG. 8 is a diagram to explain another preferred embodiment of thepresent invention.

FIG. 9 is a diagram to explain a method of detecting a cell group ID ina manner that a transmitting side transmits a specific tone boostedCAZAC sequence in a first step and that a receiving side receives theCAZAC sequence according to one embodiment.

FIG. 10 is a diagram to explain a method of detecting a cell group ID ina manner that a transmitting side transmits a specific tone boostedCAZAC sequence in a second step and that a receiving side receives theCAZAC sequence according to one embodiment.

FIG. 11A and FIG. 11B are diagrams of a signal amplitude on a frequencydomain of a code sequence transmitted according to one preferredembodiment of the present invention and an amplitude on a time domainafter completion of IFFT, respectively.

FIG. 12A and FIG. 12B are diagrams of correlation results between asignal R received according to one preferred embodiment of the presentinvention and all sequence C^(M).

FIG. 13A and FIG. 13B are diagrams of circular cross-correlationsbetween ‘99-10’ SA-CAZAC sequence and ‘199-10’ SA-CAZAC sequence onfrequency and time domains, respectively.

FIG. 14 is a diagram of a process for preamble generation and detectionusing a multi-code set according to another preferred embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The following embodiments are examples for applying technicalidea of the present invention to an OFDM (orthogonal frequency divisionmultiplexing) system.

For the explanation of the basic concept of the present invention, it isassumed that direct current (DC) and guard carrier, cyclic prefix (CP),and channel noise environments are not taken into consideration. Forfacilitation of explanation, one resource is assumed as one OFDM symbolin an OFDM system. Further, it is assumed that a sequence is inserted ona frequency domain. Zadoff-Chu CAZAC sequence is exemplarily taken as asequence to use for explanation. Alternatively, a different sequencetype having a good correlation characteristic may also be used.

FIG. 3 is a block diagram according to one preferred embodiment of thepresent invention.

Referring to FIG. 3, a SA-CAZAC sequence generating module 41 generatesa SA-CAZAC sequence by performing data processing of an embodiment thepresent invention on a specific CAZAC sequence. A sequence mappingmodule 42 maps the SA-CAZAC sequence outputted from the SA-CAZACsequence generating module 41 to a sub-carrier on a frequency domain. AnIFFT module 43 transforms frequency domain signals into time domainsignals through IFFT operation. A CP inserting module 44 inserts a guardinterval (CP: cyclic prefix). A code sequence transmitted to a receivingside via a channel is mixed with a noise in the course of transmissionand is then received by the receiving side. A CP removing module 46removes the CP from the signals received by the receiving side. Thecorresponding signals are transformed into frequency domain signals fromtime domain signals by a FFT module 47. Sequence de-mapping is thencarried out on the transformed signals by a sequence de-mapping module48. A boosted tone searching module 49 searches the de-mapped sequencefor a boosted tone. A cell ID searching module 50 obtains a cell IDusing the searched boosted tone. Examples for generating a SA-CAZACsequence by the SA-CAZAC sequence generating module 41 and a cell searchmethod by the boosted tone searching module 49 and the cell ID searchingmodule 50 are explained in detail as follows.

A k^(th) element of Zadoff-Chu CAZAC sequence for a code type index Mhaving a length N is represented as Equation 4 if N is even or Equation5 if N is odd. In generating different type sequences, there are variousmethods such as a method of performing circular shift on a CAZACsequence by considering a maximum delay spread of a channel. Forconvenience of explanation, a case of generating different sequences bydifferent M values is taken into consideration only.

$\begin{matrix}{{{{C^{M}(k)} = {\exp\left( {{\mathbb{i}}\frac{M\;\pi\; k^{2}}{N}} \right)}},{k = 0},1,2,\ldots\mspace{14mu},{N - 1}}{{when}\mspace{14mu} N\mspace{14mu}{is}\mspace{14mu}{even}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack \\{{{{C^{M}(k)} = {\exp\left( {{\mathbb{i}}\frac{M\;\pi\;{k\left( {k + 1} \right)}}{N}} \right)}},{k = 0},1,2,\ldots\mspace{14mu},{N - 1}}{{when}\mspace{14mu} N\mspace{14mu}{is}\mspace{14mu}{odd}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In case of a boosted index l according to a code type M, a code index Mis transformed in the boosted index l by Equation 6.l=index(M)  [Equation 6]

In Equation 6, ‘index( )’ indicates an index transform function and lhas one-to-one mapping relation to M values. As an example of the indextransform function, any function capable of meeting the one-to-onecorrespondence such as a linear function, a random function, and thelike is usable.

One embodiment of the present invention is characterized in boosting anl^(th) tone for a code sequence. In this case, ‘boosting an l^(th) tone’means that a power allocated to an l^(th) code element (hereinafter‘element’) of the code sequence is set greater than that of anotherelement. It is also possible for a receiving side to discriminate thel^(th) element from other elements by shifting, in a transmitting side,a phase of the l^(th) element to be different from those of otherelements as well as by the method of boosting the l^(th) tone of thecode sequence.

Moreover, it may be considered that a power allocated to an elementcorresponding to another tone except the code sequence boosted l^(th)tone is reduced lower than a predetermined level. Furthermore, it ispossible to set a power allocated to an element corresponding to anothertone except the l^(th) tone to ‘0’. In other words, it is able toconsider boosting the power allocated to the l^(th) element for the codesequence without allocating powers to other elements.

As another embodiment, it is able to consider a method of boosting atone corresponding to an element of ‘0’ using a code sequence includingone element of ‘1’ and other elements of ‘0’. For instance, a codesequence {1, 0, 0, 0, 0, 0, 0, 0, 0, 0} having a length of 10 can betransmitted by boosting the first element.

Equation 7 is provided to explain an example of a detailed method ofboosting an l^(th) tone. In the present disclosure, a specific toneboosted CAZAC sequence is defined as a single-tone added CAZAC sequence(SA-CAZAC sequence).

$\begin{matrix}{{C_{l}^{M}(k)} = \left\{ \begin{matrix}{{\frac{1}{\sqrt{\overset{\_}{P}}}\sqrt{\lambda}{C^{M}(k)}},} & {{{when}\mspace{14mu} k} = l} \\{{\frac{1}{\sqrt{\overset{\_}{P}}}{C^{M}(k)}},} & {otherwise}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

In Equation 7, C_(M)(k) is a general CAZAC sequence generated fromEquation 4 and Equation 5, ‘k’ corresponds to k=0, 1, 2, . . . , N−1,l=0, 1, . . . , N−1, and ‘M’ is a natural number relatively prime to ‘N’(e.g., M=1, 2, . . . , N−1 if N is a prime number). λ is a positive realnumber as a boosting factor. And, P is expressed as Equation 8.

$\begin{matrix}{\overset{\_}{P} = {\frac{1}{N}\left( {{\sum\limits_{k \neq l}{{C^{M}(k)}}^{2}} + {{\sqrt{\lambda}{C^{M}(k)}}}^{2}} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

In Equation 7, C_(l) ^(M)(k) indicates an l^(th) tone boosted SA-CAZACsequence and means that a power allocated to an l^(th) element isboosted λ times by multiplying an amplitude of the l^(th) element of anoriginal CAZAC sequence prior to boosting the l^(th) tone by √{squareroot over (λ)}. In Equation 8, P is to sustain a power allocated to thewhole CAZAC sequence at the same level. In particular, to boost thel^(th) tone by sustaining the power allocated to the whole CAZACsequence at the same level, powers allocated to other tones are reducedat a prescribed level.

Equation 7 and Equation 8 indicates that a procedure for boosting thel^(th) tone is carried out before the CAZAC sequence symbol is convertedto time domain signals i.e., prior to IFFT. According to anotherembodiment, it is possible to boost the l^(th) tone after modulation bysub-carriers has been performed on the CAZAC sequence through IFFT.

In this case, a boosted index l is in a one-to-one mapping relation witha code type index M but does not need to have a same value according toeach situation. For instance, if M=1, 2, . . . , N−1, a boosted indexfor each code may correspond to l=1, 2, . . . , N−1 or l=35, 2, 11, 50,. . . . For the purpose of randomization over several symbol durations,averaging, or the like, it is possible to perform hopping on a sequenceof a same code index M using a boosted index differing according to asymbol. For instance, for a code type M=10, it is possible to use l=10for a first symbol, l=20 for a second symbol, l=1 for a third symbol,and the like.

FIG. 4 is an exemplary diagram of a case that tones of symbols (l=10,l=3, l=59) are boosted in using a CAZAC sequence (M=1) for three symbolsaccording to one preferred embodiment of the present invention. By theabove-mentioned hoping method, it is able to reduce probability of cellsearch error that may be generated from deep fading that affects aboosted tone.

When C_(l) ^(M)(k) data-processed according to Equation 7 is received bythe receiving side, it can be expressed as Equation 9.R(k)=C _(l) ^(M)(k)H ^(M)(k)+N ^(M)(k)  [Equation 9]

In this case, H^(M)(k) is a value of fading that a sequence M undergoesby a k^(th) sub-carrier and N^(M)(k) is a value of AWGN (Additive WhiteGaussian Noise).

Since an l^(th) tone of C_(l) ^(M)(k) is transmitted by being boosted,the receiving side need not execute the related art complicatedoperation for calculating correlation but executes a simple FFTdemodulation process only to detect a cell ID on a frequency domain bysearching for identification information for a boosted tone, i.e., aboosted index only.

In order for the receiving side to detect a code index (cell ID) byreceiving the code sequence signal according to Equation 9, a boostedindex l′ is found using Equation 10 and then transformed into a codeindex M′ using Equation 11.l′=arg_(k){max(|R(k)|)}  [Equation 10]M′=index⁻¹(l)  [Equation 11]

In Equation 11, index⁻¹(•) indicates an index inverse transform functionand has an inverse function relation with Equation 6.

FIG. 5A and FIG. 5B are diagrams of amplitudes on frequency and timedomains for 520 sequence types generated in SA-CAZAC if Ng=521 and λ=3according to the present invention. In this case, a boosted index l foreach sequence type is equal to a code index M (i.e., l=M). In this case,total 520 sequences can be generated. In case of performing a cellsearch using an original CAZAC sequence, a complex multiplicationoperation is carried out 520×(Ng−1) times for correlation valuecomparison and a code index M having the greatest value is then detected(total 521×520 (=270920) complex multiplications executed). Yet, in caseof performing a cell search using a SZ-CAZAC sequence according to oneembodiment of the present invention, an operation of searching powers oramplitudes of 521 (Ng) tones of a received signal is necessary only.

Table 1 shows average PAPR and maximum PAPR for every sequence availableon a time domain in case of Ng=521 when a boosting factor λ varies from1 to 15 in SA-CAZAC (identical to the above example) according to oneembodiment of the present invention.

TABLE 1 Average PAPR Maximum PAPR λ [dB] [dB] 1 0 0 2 0.14787 0.147882 30.257537 0.257558 4 0.347471 0.3475 5 0.424824 0.424858 6 0.4932570.493297 7 0.554946 0.554991 8 0.611308 0.611356 9 0.663324 0.663377 100.711712 0.711768 11 0.757009 0.757068 12 0.799635 0.799697 13 0.8399230.839988 14 0.878143 0.878211 15 0.914516 0.914587

In this case, if λ=1, a sequence becomes identical to an original CAZACsequence. The λ should be selected by considering a trade-off betweenPAPR and λ. Yet, as can be seen from Table 1, a PAPR value is notconsiderably degraded from an original CAZAC sequence if Ng=1 and λ≦15.And, the PAPR value stays within an operating range to cause no problem.Of course, a case of λ>15 is not excluded.

FIG. 6A and FIG. 6B are diagrams of CDF of circular cross-correlationfor available 520 sequences according to a variation of λ (=0˜15) forNg=521 in SA-CAZAC according to one embodiment of the present invention.It can be confirmed that a SA-CAZAC sequence according to one preferredembodiment of the present invention almost has no degradation ofcross-correlation rather than an original CAZAC sequence. It is able toset a value of λ by considering a trade-off relation to correlationcharacteristic degradation for a considerably large λ. So, it can beconfirmed that the proposed sequence is usable for such a purpose as acorrelation based synchronization sequence and the like as well as acell search.

According to another preferred embodiment of the present invention, itis able to consider a 2-step cell search scheme using one resource(e.g., one OFDM symbol) by a method of boosting a specific tone of acode sequence. In particular, a plurality of cells are divided into cellgroups each of which includes at least one cell in a communicationsystem. Information for identifying a specific cell group andinformation for identifying a specific cell belonging to the specificgroup by a method of boosting a specific tone of a code sequence arethen represented. For instance, one code sequence including total N codeelements can be regarded as N different code sequences according to aboosted code element index. Namely, since a receiving side is able toclearly discriminate a code sequence having a first code element boostedfrom another code sequence having a second code element boosted, the twocode sequences can be regarded as different from each other.

For instance, if an index of a boosted code element, i.e., a boostedindex is used for cell group ID discrimination and if an originalsequence before being boosted is used for cell ID detection, the 2-stepdetection is possible. In this case, if a receiving side receives a codesequence having a specific tone boosted, the receiving side is able toperform a cell search process by searching boosted indexes, obtaining acell group ID using the searched boosted index, and then obtaining aspecific cell ID belonging to the cell group using an original sequencebefore boosting. In this case, the cell group ID matched to each boostedindex and the specific cell ID according to each code sequence are theinformation that the receiving side should store in advance therein orknow by a method of receiving the information by signaling from atransmitting side. On the other hand, it is also possible to use anoriginal sequence before boosting for cell group ID discrimination anduse identification information of a boosted code element, i.e., anboosted index for final cell ID detection.

For another embodiment, a specific CAZAC sequence is selected anddifferent tone is boosted for the selected CAZAC sequence. It is thenused as a cell search code sequence. For instance, in case that M=10 isused only in a CAZAC sequence of Ng=521, a number of cell IDs that canbe generated for a corresponding sequence amounts to 521 (0˜520). Inthis case, a boosting factor λ is set to 5. For instance, to use a sameCAZAC sequence of M=10 as a synchronization channel used by all cells, atone is differently boosted for each cell. In this case, since acorrelation value between SA-CAZAC sequences of which tones aredifferently boosted is considerably large, the sequences are regarded asthe almost same sequences. Due to this characteristic, it is able toobtain initial synchronization by the same process as using an originalCAZAC sequence. And, it is also possible to perform cell discriminationby simple amplitude comparison in a manner of transforming correspondingsignals into frequency domain signals through simple FFT demodulation.

FIG. 7A and FIG. 7B are diagrams of circular cross-correlation between asequence having 0^(th) tone boosted in a CAZAC sequence (M=10) and 520sequences having the rest of the boosted indexes. Referring to FIG. 7Aand FIG. 7B, according to a correlation result between sequences in casethat tones boosted for a same M are differentiated, it is observed thatthe sequences can be regarded as the almost same sequences. If thismethod is applied to a common SCH (synchronization channel) on which allcell use a same sequence, it is able to perform fast synchronizationacquisition. And, it is also possible to perform a cell search via theSCH.

FIG. 8 is a diagram to explain another preferred embodiment of thepresent invention. In the embodiment explained with reference to FIG. 8,a cell search is carried out by a 2-step process using at least tworesources (e.g., at least two OFDM symbols) by a method of boosting aspecific tone of a code sequence.

Referring to FIG. 8, a transmitting side, i.e., a specific celltransmits first and second code sequences to a receiving side for twoOFDM symbols, respectively. In this case, the two OFDM symbols arelocated adjacent to each other or spaced apart from each other bypredetermined symbols. The first code sequence is provided to inform thereceiving side of a cell group ID for identifying a specific cell groupincluding at least one cell, and the second code sequence is provided toindicate a cell ID for identifying a specific cell included in thespecific cell group identified by the first code sequence. A specifictone of at least one of the first and second code sequences is boostedand transmitted. Each of the first and second code sequences can betransmitted in a format of reference signals such as a preamble, amidamble, and a pilot signal.

The code sequence transmitting method can be classified into threetransmission types according to which one of the first and second codesequences will have a specific tone boosted. A first method is carriedout in a manner of boosting a specific tone of the first code sequencewithout applying boosting to the second code sequence. A second methodis carried out in a manner of boosting a specific tone of the secondcode sequence without applying boosting to the first code sequence. And,a third method is carried out in a manner of boosting specific tones ofboth of the first and second sequences.

In FIG. 8, a process for the receiving side to obtain a cell group ID bythe first code sequence is named ‘first step’ and a process for thereceiving side to obtain a cell group ID by the second code sequence isnamed ‘second step’. Assuming that there are four cell groups, it isable to identify each of the cell groups using four kinds of differentcode sequences as the first code sequences. Assuming that maximum 130cells are included in each of the cell groups, there should exist atleast 130 second code sequences. (In case of not considering the 2-stepcell search process, it is able to use 516 (=520−4) kinds of sequencesare usable as code sequences. Yet, since the 2-step search process istaken into consideration, 130 kinds of sequences are considered only.)Hence, the number of total cell IDs amounts to 520(=4*130). In thefollowing example, it is assumed that the cell group ID including thecell having transmitted the first and second code sequences is ‘2’ andthat the cell ID is ‘128’.

In case that the transmitting side transmits the first and second codesequences for the continuous or separate two symbols by one of the firstto third methods, the receiving side searches for a cell group ID byreceiving the first code sequence and then searches for a cell ID byreceiving the second code sequence, in the first step. According to thefirst to third methods, the receiving side receives the first codesequence of which specific tone is boosted (first method, third method)or the first code sequence of which specific tone is not boosted (secondmethod). Alternatively, the receiving side receives the second codesequence of which specific tone is not boosted (first method) or thesecond code sequence of which specific tone is boosted (second method,third method). A detailed operational process of the receiving side inthe first and second steps is explained as follows.

In the following embodiments, it is assumed that the first and secondcode sequences are not overlapped with each other, that an insertedsequence length is Ng=520, and that a CAZAC sequence or an SA-CAZACsequence generated from boosting a specific tone of a random CAZACsequence. In generating a CAZAC sequence of Ng=520, it is able to use amethod of generating a sequence having a prime number length of Ng′=521and discarding a last one to generate a sequence of 520. By the abovemethod, a number of usable CAZAC sequences can be increased.

First Step

(1) Case that a Transmitting Side Transmits a CAZAC Sequence of WhichSpecific Tone is not Boosted (Second Method)

In case that the transmitting side transmits a CAZAC sequence of whichspecific tone is not boosted, the receiving side detects a cell group IDusing correlations between the received CAZAC sequence and a pluralityof CAZAC sequences usable as cell group IDs already known by thereceiving side.

For instance, in case of four kinds of CAZAC sequences of cell group ID(Mg)=1, 2, 3, and 4 is used as the first code sequence to identify fourcell groups, the receiving side can detect a cell group ID (Mg′)according to Equation 12.

$\begin{matrix}{{M_{g}^{\prime} = {\arg\left\{ {\max\limits_{m}\left( {{r^{H}c^{m}}} \right)} \right\}}},{m = 1},2,3,4} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$

In this case, r is a column vector indicating a received signal andc^(m) is a CAZAC sequence having a cell group index m.

For instance, if the transmitting side transmits a CAZAC sequence ofMg=2 in the first step and if the receiving side detects a CAZACsequence of Mg′=2 from the received signal, a cell group ID indicated bythe first code sequence is ‘2’.

(2) Case that a Transmitting Side Transmits a CAZAC Sequence of WhichSpecific Tone is Boosted (First Method, Second Method)

FIG. 9 is a diagram to explain an example of detecting a cell group IDin a manner that a transmitting side transmits a specific tone boostedCAZAC sequence in a first step and that a receiving side receives theCAZAC sequence according to one embodiment.

Referring to FIG. 9, the transmitting side divides an overall tone intoa number (in this case, four) of cell group IDs in the first step,boosts a central tone of the corresponding group, and then transmits theboosted tone. FIG. 9 exemplarily shows that a tone corresponding to atone index 194 is boosted and transmitted in case that the cell group IDtransmitted by the transmitting side is ‘2’. In this case, both of thetransmitting side and the receiving side should have already knowninformation for the respective cell group ID areas by agreement inadvance or signaling.

The receiving side receives a CAZAC sequence, as shown in FIG. 9,transmitted by the transmitting side, performs demodulation thereon, andthen detects a boosted tone within a specific cell group search region.If so, the receiving side is able to search a cell group IDcorresponding to the specific cell group search region from which theboosted tone was detected. The receiving side detects the boostedspecific tone by receiving the SA-CAZAC sequence, which can be performedby simple FFT demodulation. So, an operation can be simplified. In theembodiment shown in FIG. 9, the example shows the cell group ID numberis four. Yet, in case of performing final cell ID detection in thesecond step, it is possible to provide more cell groups to perform thedetection by a simpler process.

Second Step

In the first step, it is assumed that there are four cell groups andthat each of the cell groups includes maximum 130 cells. Since the cellgroup ID is detected in the first step, the second step needs to detecta specific cell ID from maximum 130 cells included in a specific cellgroup.

(1) Case that the Transmitting Side Transmits a CAZAC Sequence of WhichSpecific Tone is not Boosted (First Method)

In the second step, if the transmitting side transmits a CAZAC sequenceof which specific tone is not boosted to enable the receiving side toretrieve a cell ID, the receiving side detects a cell ID by takingcorrelations between the received CAZAC sequence and a plurality ofCAZAC sequences known in advance. Since the receiving side obtains thecell group ID via the first step, it is able to obtain a final cell IDby finding correlation between the received CAZAC sequence and cellidentification CAZAC sequences of the cells included in the cell groupcorresponding to the cell group ID.

For instance, assuming that it is able to use 130 kinds (Mg=1, . . . ,130) of CAZAC sequences, the receiving side is able to detect a cell IDby Equation 13.

$\begin{matrix}{{M^{\prime} = {\arg\;\left\{ {\max\limits_{m}\left( {{r^{H}c^{m}}} \right)} \right\}}},{m = 5},\ldots\mspace{14mu},134} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

In this case, r is a column vector indicating a received signal andc^(m) is a CAZAC sequence having an index m.

And, the detected cell ID is used to decide a final cell ID togetherwith the cell group ID detected by the first step. For instance, if aCAZAC sequence corresponding to Mg=129 is transmitted in the second stepand if Mg′=130 is detected, a combination of a final cell ID becomes{(cell group ID in first step)-(cell ID in second step)}=(2-128).

(2) Case that Transmitting Side Transmits CAZAC Sequence of WhichSpecific Tone is Boosted (Second Method, Third Method)

FIG. 10 is a diagram to explain an example of detecting a cell group IDin a manner that a transmitting side transmits a specific tone boostedCAZAC sequence in a second step and that a receiving side receives theCAZAC sequence according to one embodiment.

Referring to FIG. 10, if an overall tone for a CAZAC sequence having anoverall length 520 is divided into 130 cell search regions, each of thecell search regions corresponds to each cell included in the specificcell group detected by the first step. The transmitting side, i.e., thecell to which the transmitting side currently belongs boosts a specifictone included in the cell search region corresponding to its cell ID andthen transmits it to the receiving side. In FIG. 10, the example showsthat a 509^(th) tone included in 128^(th) cell search region is boostedand transmitted. If the receiving side receives and demodulates theSA-CAZAC sequence, as shown in FIG. 10, it is able to detect the boostedtone. So, a number of the cell search region having the boosted toneincluded therein is detected as a cell ID.

As another preferred embodiment of the present invention, for a codesequence set including a plurality of code sequences, it is able toconsider a method of varying a position (boosted index) of a boostedtone of each of the code sequences included in the code sequence set.

For instance, although code sequence types available for a CAZACsequence of Ng=21 are total 520, since the type number of boostedindexes can be set to 521 for each of the code sequences, it is able toextend the sequence types to total 270,920 (=521×520) types. Inparticular, in case that the transmitting and receiving sides share aCAZAC sequence set of Ng=521, it is able to discriminate maximum 520cells by the CAZAC sequence if a specific tone is not boosted. Yet, byvarying a position of the boosted tone for each CAZAC sequence, it isable to discriminate maximum 270,920 cells from. Hence, it is able toextend the number of code sequences available for cell search.

In case of employing this method, the receiving side has to perform cellsearch through two steps including a boost index search and a final codesequence search. In particular, in case of receiving a code sequencehaving a randomly boosted index from the transmitting side, thereceiving side is able to detect a cell to which the receiving sideitself belongs by searching for the boosted index and then searching fora final code sequence. In this case, since one cell can be identified bya boosted index and a code index, it is able to represent a specificcell ID as a format of ‘A-M’. In this case, ‘A’ is a boosted index and‘M’ is a code index of a CAZAC sequence. In other words, ‘A-M’ indicatesa code sequence of which A^(th) tone of an M^(th) code sequence isboosted. In a CAZAC code sequence of Ng=521, ‘A’ has one of 0, 1, . . ., and 520 and ‘M’ has one of 1, . . . , and 520.

In case that the transmitting side, i.e., a specific cell boosts a99^(th) tone of a CAZAC code sequence corresponding to a code index ‘10’with a boosting factor λ (‘99-10’) and then transmits it to thereceiving side, a signal amplitude on a frequency domain of atransmitted code sequence and an amplitude on a time domain aftercompletion of IFFT are shown in FIG. 11A and FIG. 11B, respectively.

For convenience of explanation, if the receiving side receives intactsignals transmitted by the transmitting side, if signals received on atime domain are set to r (column vector), and if frequency domainsignals after FFT are set to R (column vector), the receiving side isable to detect a boosted index using Equation 10. Once the boosted indexis detected, correlation between a received code index and 520 kinds ofsequences C^(M) (M=1, 2, . . . , 520)(column vector) known in advance bythe receiving side is calculated using Equation 14. A code indexcorresponding to a greatest value is then found.

$\begin{matrix}{B^{\prime} = {\arg\left\{ {\max\limits_{M}\left( {{\frac{1}{N_{g}}R^{H}C^{M}}} \right)} \right\}}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

FIG. 12A is a diagram of a correlation result between the signals Rreceived on a frequency domain and all sequence C^(M) on the abovecondition. If the signals r received on the time domain and the timedomain value c^(M) of C^(M) are already known, the correspondingcorrelation result is shown in FIG. 12B. Namely, it is able to detect M′on a frequency domain for the received signals. And, it is also possibleto directly detect M′ without boosted index detection on a time domain.

In case that a specific tone is boosted by varying a boosted index for asame CAZAC sequence, a SA-CAZAC sequence of ‘99-10’ and a SA-CAZACsequence of ‘199-10’ does not maintain low correlation for example. FIG.13A and FIG. 13B are diagrams of circular cross-correlations between‘99-10’ SA-CAZAC sequence and ‘199-10’ SA-CAZAC sequence on frequencyand time domains, respectively. In other words, in case ofdifferentiating a position (boosted index) of a boosted tone for aspecific CAZAC sequence, separation using correlation is difficult.Namely, in a code sequence regarded as the same signal having an index‘A-M’, it is difficult to detect a value of ‘A’ through correlation.

The above-mentioned technical features of the present invention areapplicable to the currently discussed 3GPP LTE (long term evolution)system. A cell search method discussed by the 3GPP LTE can be basicallyclassified into the following three types.

1) Case of performing synchronization acquisition and cellidentification with a different SCH (synchronization channel) sequencefor each cell

2) Case that every cell performs synchronization using a same sequenceand performs cell identification by a reference signal (pilot signal)

3) Case of performing cell group identification and synchronizationacquisition using a sequence of different SCH and final cellidentification per cell group

To the above three kinds of cases, the detailed cell search methodsaccording to the preferred embodiments of the present invention areapplicable. In particular, if the technical features of the presentinvention are applied to the case 2), it is able to bring about the sameeffect as using the same sequence per cell for SCH without referencesignals.

And, a CP (cyclic prefix) for an OFDM symbol of a current LTE downlinkis supposed to use one of ‘long CP’ and ‘short CP’ within one sub-frame.A SA-CAZAC sequence according to the present invention is advantageousin finding a boosted tone index regardless of ‘short’ or ‘long’ even ifFFT is executed with reference to ‘short CP’.

The above explained technical features of the present invention areusable for other usages as well as the cell search process in the mobilecommunication system. For instance, if a transmitting side transmits acode sequence in accordance with an embodiment of the present inventionon a specific channel such as a random access channel (RACH), a controlchannel, a traffic channel, and the like, a receiving side efficientlysearches for the code sequence. So, the code sequence of the presentinvention can be used for information search on the specific channel.The receiving side is able to obtain prescribed information previouslyagreed with the transmitting side using at least one of identificationinformation for at least one boosted tone, e.g., a boosted index andidentification information for the received code sequence itself. If thereceiving side receives a code sequence of which at least two tones areboosted, it is also possible to obtain prescribed information previouslyagreed between the transmitting side and the receiving side using acombination of the identification information for the at least twoboosted tones.

FIG. 14 is a diagram of a process for preamble generation and detectionusing a multi-code set according to another preferred embodiment of thepresent invention.

One embodiment according to the present invention provides a method ofenabling a mobile station to detect a cell more efficiently. As anembodiment of the present invention, a method of detecting a cellefficiently by simplifying algorithm of a receiving side using apreamble code is provided. In order to enable a cell to be effectivelydetected, a cell combines different codes having specificcharacteristics together and then transmits the combined codes. If so, amobile station enables simpler, faster and more accurate cell searchusing the characteristics of the combined codes.

First of all, an example of a method of mixing at least two codes havingdifferent characteristics and transmitting the mixed codes is explainedas follows. If a number of code sets to use is P, the code sets arerepresented as C₁, C₂, . . . , C_(P) and each of the codes configuringthe corresponding code set can be represented as c_(j) ^(k). In thiscase, c_(j) ^(k) indicates a k^(th) code in a j^(th) code set. If eachelement of c_(j) ^(k) is represented as c_(j) ^(k)(n), a combined codecan be expressed as Equation 15.

$\begin{matrix}{{s_{k}(n)} = {\sum\limits_{j = 1}^{P}\;{w_{j}{c_{j}^{k}(n)}}}} & \left\lbrack {{Equation}\mspace{14mu} 15} \right\rbrack\end{matrix}$

In Equation 15, s_(k) is a preamble code corresponding to a k^(th) celland w_(j) is a weight for each code set. Hence, in transmitting s_(k)from the k^(th) cell, it is able to modify the weight w_(j) to transmiteach time. Namely, it is able to use a different preamble code set foreach transmission time. Although several preamble code sets are used, itis able to further reveal characteristics of a specific code received bya receiving side at a specific time by applying a different weight forthe transmission. For instance, if an easily detectible code is combinedwith a preamble code and is then transmitted, a mobile station is ableto detect a cell by detecting a code set signal transmitted at aspecific time without retrieving all code sets. And, code orsynchronization information can be substantially acquired usingdifferent code set information.

Referring to FIG. 14, a transmitting side combines codes (31-1˜31-p)respectively selected from P code sets different from each other incharacteristics (33) and then transmits the combined codes. In thiscase, by giving a corresponding weight to each code set (32), it is ableto further reveal a characteristic of a code having a specificcharacteristic. A receiving side having received the combined codesperforms a first code index detection (34). For instance, it is able todetect a code index using a line search using DFT and correlation. Codere-detection is then performed using a second code (35). Signalprocessing is performed using the detected code index (36). Forinstance, cell search and time and frequency synchronizations can beperformed.

In the following description, as an example of the code combination set,a combination between a CAZAC sequence and a single tone sequence isexplained. If C₁ is a CAZAC sequence code set, C₁ includes codes shownin Equation 16.

$\begin{matrix}{C_{1} = \begin{bmatrix}{\left\{ {{\exp\left( \frac{{\mathbb{i}}\;{\pi 101}}{M} \right)},{\exp\left( \frac{{\mathbb{i}}\;{\pi 112}}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{{\mathbb{i}\pi 1}\left( {M - 1} \right)}M}{M} \right)}} \right\},\left\{ {{\exp\left( \frac{\mathbb{i}\pi 201}{M} \right)},} \right.} \\{\left. {{\exp\left( \frac{\mathbb{i}\pi 212}{M} \right)},\ldots\mspace{11mu},{\exp\left( \frac{{{\mathbb{i}\pi 2}\left( {M - 1} \right)}M}{M} \right)}} \right\},\ldots\mspace{14mu},} \\\left\{ {{\exp\left( \frac{{{\mathbb{i}\pi}\left( {M - 1} \right)}01}{M} \right)},{\exp\left( \frac{{{\mathbb{i}\pi}\left( {M - 1} \right)}12}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{{\mathbb{i}\pi}\left( {M - 1} \right)}\left( {M - 1} \right)M}{M} \right)}} \right\}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 16} \right\rbrack\end{matrix}$

In Equation 16, a first code c₁ ¹ belonging to C₁ is

${\exp\left( \frac{{\mathbb{i}\pi}\; 101}{M} \right)},{\exp\left( \frac{\mathbb{i}\pi 112}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{{\mathbb{i}\pi 1}\left( {M - 1} \right)}M}{M} \right)},$a second code c₁ ² belonging to C₁ is

${\exp\left( \frac{\mathbb{i}\pi 201}{M} \right)},{\exp\left( \frac{\mathbb{i}\pi 212}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{{\mathbb{i}\pi 2}\left( {M - 1} \right)}M}{M} \right)},$and a last code c₁ ^(M) belonging to C₁ is

${\exp\left( \frac{{{\mathbb{i}\pi}\left( {M - 1} \right)}01}{M} \right)},{\exp\left( \frac{{{\mathbb{i}\pi}\left( {M - 1} \right)}12}{M} \right)},\ldots\mspace{14mu},{{\exp\left( \frac{{{\mathbb{i}\pi}\left( {M - 1} \right)}\left( {M - 1} \right)M}{M} \right)}.}$In this case, ‘M’ indicates a code length.

If C₂ is a single tone code set, C₂ includes codes according to Equation17.

$\begin{matrix}{C_{2} = \begin{bmatrix}{\left\{ {{\exp\left( \frac{{\mathbb{i}}\; 2{\pi 00}}{M} \right)},{\exp\left( \frac{{\mathbb{i}2}\;{\pi 01}}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{\mathbb{i}2\pi 0}\left( {M - 1} \right)}{M} \right)}} \right\},\left\{ {{\exp\left( \frac{\mathbb{i}2\pi 10}{M} \right)},} \right.} \\{\left. {{\exp\left( \frac{\mathbb{i}2\pi 11}{M} \right)},\ldots\mspace{11mu},{\exp\left( \frac{{\mathbb{i}2\pi 1}\left( {M - 1} \right)}{M} \right)}} \right\},\ldots\mspace{14mu},} \\\left\{ {{\exp\left( \frac{{{\mathbb{i}2\pi}\left( {M - 1} \right)}0}{M} \right)},{\exp\left( \frac{{{\mathbb{i}2\pi}\left( {M - 1} \right)}1}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{{\mathbb{i}2\pi}\left( {M - 1} \right)}\left( {M - 1} \right)}{M} \right)}} \right\}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 17} \right\rbrack\end{matrix}$

In Equation 17, a first code c₂ ¹ belong into C₂ is

${\exp\left( \frac{\mathbb{i}2\pi 00}{M} \right)},{\exp\left( \frac{\mathbb{i}2\pi 01}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{\mathbb{i}2\pi 0}\left( {M - 1} \right)}{M} \right)}$a second code c₁ ² belonging to C₂ is

${\exp\left( \frac{\mathbb{i}2\pi 10}{M} \right)},{\exp\left( \frac{\mathbb{i}2\pi 11}{M} \right)},\ldots\mspace{14mu},{\exp\left( \frac{{\mathbb{i}2\pi 1}\left( {M - 1} \right)}{M} \right)},$and a last code c₁ ³ belonging to C₂ is

${\exp\left( \frac{{{\mathbb{i}2\pi}\left( {M - 1} \right)}0}{M} \right)},{\exp\left( \frac{{{\mathbb{i}2\pi}\left( {M - 1} \right)}1}{M} \right)},\ldots\mspace{14mu},{{\exp\left( \frac{{{\mathbb{i}2\pi}\left( {M - 1} \right)}\left( {M - 1} \right)}{M} \right)}.}$In this case, ‘M’ indicates a code length.

If signals received by the receiving side are interpreted, it is givenby Equation 18.s _(fk) =Fs _(k) =Fw ₁ c ₁ ^(k) +Fw ₂ c ₂ ^(k) +Fa  [Equation 18]

In Equation 18, ‘F’ indicates a Fourier transform matrix and ‘a’indicates a noise vector.

A CAZAC sequence has the feature shown in Equation 19.|c _(l) ^(k)(n)|=1 and c _(f1) ^(k)(n)|=x ₁  [Equation 19]

Meanwhile, a single tone sequence has the feature shown in Equation 20.

$\begin{matrix}{{{c_{2}^{k}(n)}} = {{1\mspace{14mu}{and}\mspace{14mu}{{c_{f\; 2}^{k}(n)}}} = \left\{ \begin{matrix}{M,{n = k}} \\{0,{n \neq k}}\end{matrix} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 20} \right\rbrack\end{matrix}$

In Equation 19 and Equation 20, c_(f1) ^(k) and c_(f2) ^(k) result fromtransforming the CAZAC sequence and the single tone sequence on afrequency domain, respectively. In particular, the CAZAC sequence CAZACis a constant having a uniform amplitude on time and frequency domains.The single tone sequence is a constant on a time domain but has animpulse (Dirac Delta) function format on a frequency domain. Hence, anamplitude of a frequency domain signal on a time domain can be expressedas Equation 21.|s _(fk) |=|Fw ₁ c ₁ ^(k) +Fw ₂ c ₂ ^(k) +Fa|<|c _(f1) ^(k) |+|c _(f2)^(k) |+|Fa|  [Equation 21]

Yet, if n≠k, it is |c_(f1) ^(k)|>>|c_(f2) ^(k)|. If n=k, it is |c_(f1)^(k)|>>|c_(f2) ^(k)|. Since |Fa| is a very small value, Equation 21 canbe expressed as Equation 22.|s _(fk) |≈|c _(f1) ^(k) |+|c _(f2) ^(k) |+|Fa|  [Equation 22]

Accordingly, if Equation 22 indicating a signal on a frequency domain isreferred to in order to find a received preamble code, it is able torecognize what is the transmitted code via the information obtained froma single tone. Since a value in case of n=k is considerably differentfrom a value in case of n≠k, it is able to find the transmitted codeusing a maximum value. A method of finding an index of a transmittedcode using a single code is represented as Equation 23.

$\begin{matrix}{K^{*} = {\arg\underset{{n = 0},1,\ldots\mspace{14mu},{M - 1}}{\;\max}{{s_{fk}(n)}}}} & \left\lbrack {{Equation}\mspace{14mu} 23} \right\rbrack\end{matrix}$

Hence, an index K* of a transmitted CAZAC sequence is found using thevalue detected by the above method. It is then able to acquire time andfrequency synchronization using this. Actually, transmitted signals areactually distorted or frequency synchronization is not matched betweentransmission and reception as soon as a transmission filter is added.So, a pulse is not detected from a single position of the receivedsignals. In case of searching codes, it is reasonable that detectionshould be attempted for another code in the vicinity of a position of apeak detected according to Equation 23.

A cost function for detecting a pulse from several positions isexpressed as Equations 24 to 26. First of all, a correlation value witha received signal is calculated for other codes in the vicinity of thepeak detected by Equation 23.

$\begin{matrix}{{R_{q}(\tau)} = {\sum\limits_{n = 0}^{M - 1}\;{{s_{k}(n)}{c_{1}^{q}\left( {n + \tau} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 24} \right\rbrack\end{matrix}$

In Equation 24, R_(q)(τ) is a correlation value with a q^(th) code. Inthis case, ‘q’ may become the value K* determined by Equation 23 or oneof a plurality of different values in the vicinity of the value K*.

Like Equation 25, peaks in the vicinity of the position of the peakdetected by Equation 23 are searched using a maximum value of thecorrelation value.J(q)=max R _(q)(τ)  [Equation 25]

Like Equation 26, a maximum peak value is found among values resultingfrom multiplying peak values detected by Equation 25 by the peak valuesdetected by

Equation 23.

$\begin{matrix}{K^{*} = {\arg\underset{q}{\;\max}{{s_{fk}(q)}}{J(q)}}} & \left\lbrack {{Equation}\mspace{14mu} 26} \right\rbrack\end{matrix}$

It is able to obtain an index K* of a preamble code using the codecorresponding to the maximum peak value. It is then able to searchneighbor cells using it.

In this case, instead of multiplying peak values detected by Equation 25by the peak values detected by Equation 23, addition or other operationsare executed. The transmitted preamble code is then found using amaximum value.

Meanwhile, like Equation 15, since a power is scattered by the weightw_(j), distributed powers should be taken into consideration incalculating time and frequency synchronization. Hence, it is able tomaintain synchronization performance only if algorithm is executed usingthe two codes.

Besides the transmission of the different code type combination, byperforming a differently weighted transmission (time differencetransmission using independent codes), it is able to perform simpler andmore efficient cell search.

While the present invention has been described and illustrated for thepurpose of cell search through code sequence modifications according tothe technical features of the present invention, it will be apparent tothose skilled in the art that the technical features of the presentinvention are applicable to such a communication system functionperformed using the code sequence as initial synchronization, time andfrequency synchronization acquisition, channel estimation, and the like.

Accordingly, the present invention provides the following effects.

First of all, a receiving side, i.e., a terminal is able to detect acell ID by simple amplitude comparison only after FFT demodulation,whereby complexity of the receiving side for cell search can be reduced.

Secondly, by reducing a cell search time, fast initial acquisition ispossible.

Thirdly, fast cell search for handover is possible.

Fourthly, detection error probability in cell search is lowered.

Fifthly, by using common SCH to acquire fast initial acquisition andperforming cell discrimination with it, the present invention achievestwo objects of resource saving and fast cell search.

Sixthly, sequence types available for cell search can be extended.

Seventhly, in an OFDM symbol that used at least tow kinds of CP lengths,it is able to perform cell search regardless of the CP length.

Eighthly, despite providing the above effects, degradation of majorcharacteristics (good correlation, low PAPR, etc.) of the original CAZACsequence is prevented.

While the present invention has been described and illustrated hereinwith reference to the preferred embodiments thereof, it will be apparentto those skilled in the art that various modifications and variationscan be made therein without departing from the spirit and scope of theinvention. Thus, it is intended that the present invention covers themodifications and variations of this invention that come within thescope of the appended claims and their equivalents.

INDUSTRIAL APPLICABILITY

Accordingly, the present invention is applicable to a wirelesscommunication system such as a mobile communication system, a wirelessinternet system, and the like.

What is claimed is:
 1. A method of transmitting a synchronizationsignal, the method comprising: generating a first code sequence from afirst number which is related to a cell group identifier for identifyinga cell group including the cell and a second code sequence from a secondnumber which is related to a cell identifier for identifying the cellincluded in the cell group identified by the cell group identifier,wherein a combination of the first number and the second numberindicates an identity of the cell; and transmitting the first codesequence via a first orthogonal frequency-division multiplexing (OFDM)symbol and the second code sequence via a second OFDM symbol.
 2. Themethod of claim 1, wherein the first OFDM symbol is ‘n’ OFDM symbolahead from the second OFDM symbol.
 3. The method of claim 2, wherein the‘n’ is 1.